Methods for forming directly a positive image using a color diffusion transfer process include (A) a method wherein direct positive silver halide emulsions are used in combination with a nondiffusible compound (called a negative dye providing compound) which releases a diffusible dye in relation to a reaction which reduces silver halide to silver; and (B) a method wherein general silver halide emulsions (silver halide emulsions which undergo negative-positive response) are used in combination with a nondiffusible compound which itself becomes diffusible in counter-relation to a reaction which reduces silver halide to silver, or a nondiffusible compound (called a positive dye providing compound) which releases a diffusible dye in counter-relation to a reaction which reduces silver halide to silver.
In method (A), compounds (DDR couplers) which are couplers having a diffusible dye as a split-off group and release a diffusible dye by the coupling reaction with the oxidation products of reducing agents as described in U.K. Patent 1,330,524, JP-B-48-39165 (the term "JP-B" as used herein means an "examined Japanese patent publication") and U.S. Pat. Nos. 3,443,940, 4,474,867 and 4,483,912; and compounds (DRR compounds) which are capable of reducing silver halide and release a diffusible dye when silver halide is reduced as described in U.S. Pat. Nos. 3,928,312, 4,053,312, 4,055,428 and 4,336,322 are used.
In method (B), the following compounds are used:
1 dye developing agents wherein a hydroquinone developing agent and a dye component are bonded to each other (the dye developing agents are diffusible under alkaline conditions, but become nondiffusible when reacted with silver halide) as described in U.S. Pat. Nos. 3,134,764, 3,362,819, 3,597,200, 3,544,545 and 3,482,972;
2 nondiffusible compounds which release a diffusible dye under alkaline conditions, but lose the ability to release the dye when reacted with silver halide as described in U.S. Pat. No. 4,503,137, compounds which release a diffusible dye by an intramolecular nucleophilic displacement reaction as described in U.S. Pat. No. 3,980,479, and compounds which release a diffusible dye by the intramolecular rewinding reaction of isoxazolones as described in U.S. Pat. No. 4,199,354; and
3 nondiffusible compounds which release a diffusible dye by the reaction with a reducing agent remaining without being oxidized by development as described in U.S. Pat. No. 4,559,290, European Patent 220,746A2, U.S. Pat. Nos. 4,783,396 and Kokai Giho 87-6199.
When the above two methods are compared, method (B) is preferable from the viewpoint of easily achieving high sensitivity. However, method (B) has a problem in that there is a difficulty in reducing the density of the minimum density area which is of great importance for image formation.
In method (B), the minimum density of the positive image is determined by a competitive reaction between the dye release due to the reaction of a reducible dye providing compound with an electron donor and the oxidation of the electron donor by the oxidation product (formed by development of silver halide) of an electron transfer agent. Accordingly, the formation of the oxidation product of the electron transferring agent is appropriately adjusted by controlling the development of light-sensitive silver halide to a lower minimum density. Specifically, a light-sensitive silver halide which can be rapidly developed is used to expedite the formation of the oxidation product of the electron transferring agent, and the use of a precursor of the electron donor has been disclosed (see, JP-A-3-131848 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")). However, these methods are methods which have an effect on dye release which occurs at an early stage of development. It has been found that the minimum density of a sufficiently satisfactory positive image can not be achieved only by these methods. The present invention is directed to a method having an effect on dye release which occurs at a latter stage of development.